feat(fase-2b): Realtime sync + offline queue + Modo Compra

Fase 2b closes the critical-path product differentiator. Two sessions on the
same list see each other's changes in real time, mutations survive network
drops and sync when reconnected, and a dedicated full-screen shopping view
optimises the in-store experience.

2b.1 Realtime
- `$lib/stores/realtimeSync.ts` wraps `postgres_changes` with an `applyItemEvent`
  reducer (INSERT/UPDATE/DELETE → next items[]) and a `subscribeToList` helper
  that awaits the SUBSCRIBED handshake before returning
- `/lists/[id]` subscribes in onMount, unsubscribes in onDestroy. Uses a
  `pendingTempIds` set to deduplicate own-mutation echoes against optimistic
  rows (matches by name+created_by+sort_order)
- Client-generated UUIDs for new inserts make the path idempotent: if the
  server response is lost and we retry, the second POST hits PK-conflict
  which the queue treats as success
- CHECKED section div now carries role="listitem" so Playwright locators
  follow the item across sections

2b.2 Offline queue
- `$lib/sync/queue.ts` — SyncQueue class backed by IndexedDB (idb), FIFO flush,
  MAX_ATTEMPTS=5 retry budget, PK-conflict-as-success short-circuit
- `$lib/sync/index.ts` — app-wide singleton, window.online listener flushes
  automatically, hydrateSyncState() at page load restores pendingOpsCount
- `$lib/stores/syncStatus.ts` — derived store (offline | syncing | synced)
  tracking navigator.onLine + queue depth
- SyncBanner component renders the offline/syncing indicator in the detail
  and session views
- handleAdd enqueues on failure instead of reverting, so an offline mutation
  keeps its optimistic row and syncs on reconnect

2b.3 Modo Compra
- `/lists/[id]/session/+page.svelte` — full-screen overlay (fixed inset-0
  z-50) that covers the sidebar while keeping the normal auth routing.
  56×56 toggles, flip animation shuffling items between TO BUY / CHECKED,
  Finish Shopping confirmation modal → completeList → goto('/lists')
- Link from `/lists/[id]` header (data-testid=start-session) with the
  new `list_start_session` message

Testing infra
- apps/web gets its own Vitest config (jsdom + fake-indexeddb/auto) and a
  new `pnpm test:unit` script. `just test-all` now chains pgTAP → integration
  → unit → e2e so a single command is the gate.
- packages/test-utils/tests/sync-queue.test.ts (placeholder scaffold) removed —
  replaced by `apps/web/src/lib/sync/queue.test.ts` co-located with the module

Totals: 103 tests green
  16 pgTAP
  59 Vitest integration (in packages/test-utils)
   6 Vitest unit (in apps/web — the SyncQueue)
  22 Playwright E2E (5 auth + 4 lists + 6 items + 2 realtime + 2 offline + 3 session)
   2 skipped (realtime-presence — upstream bug, unchanged)

Documented in plan/fase-2b and CLAUDE.md.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

apps/web/vitest.setup.ts

@@ -0,0 +1,3 @@
+// Polyfill IndexedDB for tests. Node's jsdom env doesn't ship with IDB.
+// Imported at file top so the shim is in place before any test code runs.
+import 'fake-indexeddb/auto';